An incredible news story today is about a man who survived for two days at the bottom of the sea (~30 m deep) in a capsized boat, in an air bubble that formed in a corner of the boat. He was eventually rescued by divers who came to retrieve dead bodies. Details here. Since gases diffuse through water (and are dissolved in it) the composition of air in the bubble should be close to the atmosphere outside, if the surface of the bubble is large enough; so the excessive carbon dioxide is removed and oxygen is brought in to support life of a human.

Question: How large does the bubble have to be so that a person in it can have indefinite supply of breathable air?

On a second thought, an air pocket under water seems to be a situation out of thermodynamic equilibrium; to minimize the potential energy of the system the air in the pocket should dissolve in the water so that the water would fill the pocket and lower its center of gravity. However it is known for a fact that air pockets can exist in underwater caves (newton.dep.anl.gov/askasci/env99/env99438.htm) - how is that possible?
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Maxim UmanskyJun 14 '13 at 4:59

Someone should do an experiment to test this using a machine that consumes oxygen and produces carbon dioxide at about the same rate as a human, measuring gas concentrations throughout. Very interesting question.
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02019Jun 14 '13 at 20:44

3 Answers
3

Summary: I find a formula for the diameter of a bubble large enough to support one human and plug in known values to get $d=400m$.

I'll have a quantitative stab at the answer to the question of how large an air bubble has to be for the carbon dioxide concentration to be in a breathable steady state, whilst a human is continuously producing carbon dioxide inside the bubble.

Fick's law of diffusion is that the flux of a quantity through a surface (amount per unit time per unit area) is proportional to the concentration gradient at that surface,

$$\vec{J} = - D \nabla \phi,$$

where $\phi$ is concentration and $D$ is the diffusivity of the species. We want to find the net flux out of the bubble at the surface, or $\vec{J} = -D_{\text{surface}} \nabla \phi$.

$D_{\text{surface}}$ is going to be some funny combination of the diffusivity of $CO_2$ in air and in water, but since the coefficient in water is so much lower, really diffusion is going to be dominated by this coefficient: it can't diffuse rapidly out of the surface and very slowly immediately outside the surface, because the concentration would then pile up in a thin layer immediately outside until it was high enough to start diffusing back in again. So I'm going to assume $D_{\text{surface}} = D_{\text{water}}$ here.

To estimate $\nabla \phi$, we can first assume $\phi(\text{surface})=\phi(\text{inside})$, fixing $\phi(\text{inside})$ from the maximum nonlethal concentration of CO2 in air and the molar density of air ($=P/RT$); then assuming the bubble is a sphere of radius $a$, because in a steady state the concentration outside is a harmonic function, we can find

Next we integrate this over the surface of the bubble to get the net amount leaving the bubble, and set this $=$ the amount at which carbon dioxide is exhaled by the human, $\dot{N}$. Since for the above simplifications $J$ is constant over the surface (area $A$), this is just $JA$.

So we have
$$\dot{N} = D_{\text{water}} A \frac{\phi(\text{inside})-\phi(\text{far})}{a} = D_{\text{water}} 4 \pi a (\phi(\text{inside})-\phi(\text{far})).$$

Finally assuming $\phi(\text{far})=0$ for convenience, and rearranging for diameter $d=2a$

EDIT: It's interesting to note that this is independent of pressure: I've neglected pressure dependence of $D$ and human resilience to carbon dioxide, and the maximum safe concentration of carbon dioxide is independent of pressure, just derived from measurements at STP.

Finally, a bubble this large will probably rapidly break up due to buoyancy and Plateau-Rayleigh instabilities.

+1 for calculating. So my estimate of about 8*8*3 meters of surface air trapped dominates the reason he was saved. a diameter of 400 meters is no go on a tugboat.
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anna vJun 14 '13 at 19:13

I don't see any mention of pressure - at 30m CO2 dissolves into water much much faster. Edit: Never mind. But can you explain why pressure doesn't matter? I've made seltzer - it dissolves MUCH faster at higher pressure.
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ArielJun 14 '13 at 19:16

@annav Sure. A pocket of air in a boat of that diameter won't have a large air-water interface for carbon dioxide to diffuse between the water and the air, anyway.
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Chay PatersonJun 14 '13 at 19:17

@Ariel Pressure dependence of $D$ might matter, I've just neglected it due to lack of handy data. It would be pretty straightforward to plug into that formula for a revised estimate, though.
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Chay PatersonJun 14 '13 at 19:18

Note that due to convection, much smaller diameters are realistic in practice. In the (unrealistic) best case, $ D_{surface} $ may approach $ D_{air} $, which is about 4 orders of magnitude larger than $ D_{water} $. This gives $ d = 0.4 m $.
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Christian AichingerMar 9 at 21:26

The physics of the situation is the same as when one can upturn a water glass with the water not falling out. The atmospheric pressure keeps it in.

There exist the diving bells with open bottoms . As they are lowered the pressure in the air goes up to balance the water pressure, because the lower in the water the higher the pressure. I think one atmosphere is ten meters of water. The person in the link was at 30 meters and that is why he had to be decompressed in order not to get the bends.

The oxygen versus CO2 and volume to survive one day is solved in this link:

The atmosphere is about 20 percent oxygen. People breathe this in, and breathe 15 percent oxygen out, making the air that's left lower in Good Ol' Oh Two, but still quite breathable. Each minute a person at rest takes in roughly seven to eight liters of air, which adds up to about 11,000 liters of air a day. That sounds like a lot, volume-wise it's only 388 cubic feet. A ten by ten by ten foot room has 1000 cubic feet of air.Add that to the fact that you'll be breathing out a lot of oxygen, and you only need about 19 cubic feet of pure oxygen a day. Your breathing may get labored by the end of the second day, but a relatively small room should be fine for about three days. right?

wrong because there is CO2 exhaled which at large concentrations is poisonous:

Once the carbon dioxide levels in the room rise to two percent, carbon dioxide poisoning starts to happen. That occurs when the overall oxygen level falls to 19 percent, about half way through the first day.

For the person to survive three days the compressed air he was breathing must have been from a lot of volume. From these numbers an order of magnitude estimate for non renewable volume to survive for 3 days is 6000 cubic feet. An enclosure of 10*25*25 in feet is a reasonable one on a tugboat. He was just lucky the air siphoned where he was trapped.

This is not what the question is about. In diving bell, the air is supplied from the surface. Here, the point is that the only air exchange happened between the air in the bubble and the water surface. The bubble was (presumably) too small to have enough oxygen for two days. How large should the surface be to support one human?
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gigacyanJun 14 '13 at 7:30

2

To "anna v" - Yes, this is same as a diving bell, that's clear. However, the question is whether in such a diving bell one can have indefinite supply of breathable air since the air in the bell communicates through the water with the atmosphere. A human needs 10 cubic meter of air per day. That guy somehow survived for ~2.5 days.
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Maxim UmanskyJun 14 '13 at 7:31

42

Using imperial measures in a discussion on Physics?
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FedericoJun 14 '13 at 8:38

CO2 dissolves very well in water. Especially cold water. So the question is, how large of a surface area of water do you need for the CO2 to dissolve into the water as fast as the human makes it? (Assuming the water is constantly replenished.) The pressure, i.e. depth matters a lot here - the questioner specified 30m. It may be hard to answer this because mixing will significantly increase the rate - so how fast is the water flowing under the bubble, and does it make any waves or foam.
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ArielJun 14 '13 at 9:02

This is to summarize some of the excellent comments made
previously by participants of this discussion, and to emphasize
a couple of important points.

1) The original question implied that gas exchange between the bubble
and surrounding water may be enough to sustain indefinitely
breathing organisms inside. However this does not seem to be
possible if the bubble is below the sea level because the gas in
the bubble will eventually dissolve in the water and the bubble will
cease to exist. The reason is that the concentration of dissolved
gas near the bubble surface has to be higher than the ambient
concentration. The ambient concentration is not higher than its
value at the ocean surface (it is convective mixing rather than
diffusion what transports dissolved air into the ocean). However
the bubble is at the pressure $\sim$4 times the atmospheric
pressure, and according to the Henry law the dissolved gas
concentration next to the bubble should be $\sim$4 times higher
than the ambient (the possible variation of temperature does not
change the overall conclusion). So eventually the dissolved gas
will diffuse away and the bubble will disappear. Thus, for any size
of the bubble, with or without breathing organisms inside, there is
no steady-state solution, if the bubble is below the sea
level. This is illustrated in the figure: for either regular or
upside-down siphon, the equilibrium state has water level equal in
both legs. If one leg end in Fig. A is below the equilibrium level then
eventually there will be no gas on this side of the siphon; this is similar
to having no water in one leg in Fig. B if the end of this leg is above the
equilibrium level.

2) The good news is that it would take a long time for the bubble to
diffuse away. Estimate the diffusion time as

$
\tau \sim \frac{n_{g}}{c} \frac{a^2}{D},
$

where $c/n_g$ is the ratio of concentrations of dissolved gas molecules
and gas molecules in the
bubble, $a$ is the size of the bubble, $D$ is the diffusion
coefficient of dissolved gas.

According to Wikipedia, at 25 C, $c/n_g$ values are:
for N$_2$ $1.492\times10^{-2}$; for O$_2$ $3.181\times10^{-2}$;
for CO$_2$ $0.8317$.

So, with these numbers, for an air bubble of the size $a\sim$ 1 m
the lifetime is $\sim 10^{11}$ s $\sim$10,000 years! This is just
unrealistic (if such things were possible we would've known about
that!). The problem with this estimate is that the gas diffusion
coefficient in water,
D$_g\sim$10$^{-5}$cm$^2$/s = 10$^{-9}$m$^2$/s applies only to
phenomena on small spatial scales, e.g., transport in porous
media. We know that we can make carbonated water at home in a
matter of minutes rather than months! On macroscopic scales
transport is dominated by turbulence and convective flows, and,
rather than diffusion, macroscopic transport models use the notion
of effective ``exchange velocity'' $V_{ex}$, such that the flux of
dissolved gas is

$
\Gamma = A c V_{ex},
$

where $A$ is the area, $c$ the concentration of dissolved gas.

The idea is that dissolved gas is removed by background convection
and/or turbulence at the rate $V_{ex}$. Typical magnitude of
$V_{ex}$ in the ocean, near the surface, can be 1 m/day
[reference needed].

which, for $V_{ex}$=1 m/day, yields $\tau\sim$100 days, much more
reasonable that 10,000 years!

3) As pointed out earlier, probably one does not need to make complex
assumptions about water flows,
turbulent diffusion etc if the available bubble of
air contained the equivalent of $\sim$25 $m^3$ at atmospheric
pressure, now compressed into $25/4\sim$6 m$^3$ volume,
theoretically that would be enough for 2.5 days, without degassing
excessive CO$_2$ into the water, or importing O$_2$ from
surrounding water (the latter is impossible anyway as discussed in
(1) above). The way it is described in the media, the man
``...survived, breathing inside a four-foot high bubble of air as
it slowly shrank from the waters rising from the ceiling of the
tiny toilet and adjoining bedroom'', but probably that four-foot
bubble communicated with a larger volume or air under the hull of
the boat - and that's the most reasonable explanation of this
miraculous survival.

4) One can note that due to different solubility of CO$_2$, N$_2$, and
O$_2$ these gases will be leaving the bubble at different rates,
and this is perhaps good news. Due to the breathing human present
inside, the CO$_2$ level in the bubble will be increasing and the
O$_2$ level will be accordingly decreasing. Once this goes to the
dangerous level, say 5$\%$ CO$_2$ and 15$\%$ O$_2$, it will be
mostly CO$_2$ that will be leaving the bubble, transport of other gases will
be much slower. As pointed out by other people in this discussion, with
the ``classical'' diffusion the gas removal rate would not be
enough to counterbalance the buildup of CO$_2$ (unrealistic surface area
needed), but background flows and turbulence in water can significantly increase the
rate of transport, and perhaps the available surface area as well
(foam formation).

To make a quantitative estimate, let's take that the human rate
of O$_2$ consumption and CO$_2$ production is $\sim 10^{25}$
molecules/day; and for the concentration of dissolved carbon
dioxide use

assuming transport of dissolved CO$_2$ with the exchange velocity
$V_{ex}$=1 m/day, and one can see that just a couple of square meters of
surface area would be enough to balance the buildup of exhaled CO$_2$
concentration. This effect can probably extend the survival time by
a factor of $\sim$2. Normally only a quarter of available oxygen in
the air can be used, reducing it from 20$\%$ on inhale to 15$\%$ on
exhale. With excessive CO$_2$ removed from the air by water, however, probably surviving on the fraction of oxygen in the air as low as 10$\%$ is possible.
For example, the diving gas mix Heliox uses 10$\%$ oxygen (and the rest helium).

5) And last, it is interesting to note that in principle in this kind
of situation one could possibly survive free ascent on a single breath from
this depth; in the
history of submarine accidents some people managed to escape from
sunken submarines to the surface even from greater depths by free
ascent. Of course the risk of doing so would be enormous; but
probably not greater than the risk of not to be found in time.

@annav I'm pretty sure this answer is spot-on. The fact is bubbles are thermodynamically unstable, and the pathway bubble $\to$ no bubble is kinematically possible due to convective mixing, both at the interfaces and throughout the water.
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Chris WhiteJun 24 '13 at 5:58

2

In addition with this logic I do not see why earth's atmosphere has not been dissolved in the oceans over the millenia. After all the measure 1 atmosphere is arbitrary. It is the equal pressure in the water and in the gas: both at one atmosphere at the surface, both at 4 atmospheres in the ocean so your argument would have all the atmosphere of the earth in contact with the surface ocean be absorbed.
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anna vJun 24 '13 at 8:11

1

@annav your example assumes the ocean won't reach a saturation limit for gasses. this bubble in the ocean example the gas volume is very small compared the water.
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user6972Jun 25 '13 at 20:02

1

@user6972, if the ocean reaches the saturation limit, isn't that also applicable to bubbles and pockets of gases? Even though bubbles are thermodynamically unstable, if saturation is reached in the ocean then it will not dissolve any more.
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ChrisRJun 26 '13 at 8:42

2

I am saying that the data say that air can be trapped a long time under water. I am too rusty ( retired, 73yrs old) to really look at the thermodynamics of the answer ( not my field anyway). There must be some factor that has been forgotten, since data trumps calculations everytime.
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anna vJun 29 '13 at 15:31